Energetics

• What happens during an enthalpy change?

  Energy is transferred between system and surroundings
• What defines the system and surroundings in a chemical reaction?

  The system is chemicals; surroundings are everything else
• In an exothermic change, where is energy transferred?

  From the system to the surroundings
• What is the sign of ∆H in an exothermic reaction?

  Negative
• In an endothermic change, where is energy transferred?

  From the surroundings to the system
• What is required for an endothermic reaction?

  An input of heat energy
• What is the sign of ∆H in an endothermic reaction?

  Positive
• What are the standard conditions for enthalpy changes?

  • 100 kPa pressure
  • 298 K (25°C)
  • Solutions at 1 mol dm^-3
  • All substances in their normal state at 298 K
• What symbol is used for enthalpy change measured at standard conditions?

  ΔH
• What is the standard enthalpy change of formation?

  Enthalpy change when 1 mole forms from elements
• What is the symbol for standard enthalpy change of formation?

  ΔfH
• Write the reaction for the formation of magnesium chloride.

  Mg (s) + Cl₂ (g) → MgCl₂ (s)
• Write the reaction for the formation of iron(III) oxide.

  2Fe (s) + 1.5 O₂ (g) → Fe₂O₃ (s)
• What is the enthalpy of formation of an element?

  0 kJ mol^-1
• What is the standard enthalpy change of combustion?

  Enthalpy change when one mole combusts in oxygen
• What symbol is used for standard enthalpy change of combustion?

  ΔcH
• Write the reaction for the combustion of methane.

  CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2 H₂O (l)
• What happens during incomplete combustion?

  Produces soot, carbon monoxide, and water
• What are common exothermic processes?

  Combustion of fuels and oxidation of glucose
• What is the definition of enthalpy change?

  Heat energy change during a system change
• What is the calorimetric method for measuring enthalpy change?

  • Measure temperature change of reactants
  • Use energy change equation: Q = m x cp x ΔT
  • Extrapolate temperature curve for slow reactions
• What is the equation for calculating energy change in a reaction?

  Q = m x cp x ΔT
• What does 'm' represent in the energy change equation?

  Mass of solution in grams
• What does 'cp' represent in the energy change equation?

  Heat capacity in J g^-1 K^-1
• What does 'ΔT' represent in the energy change equation?

  Temperature change in Kelvin
• What is the first step in calculating enthalpy change from experimental data?

  Calculate energy change using Q = m x cp x ΔT
• How do you calculate the number of moles of a reactant?

  Moles = concentration x volume
• How do you convert energy change to enthalpy change per mole?

  Divide energy change by moles of limiting reactant
• What is the heat capacity of water?

  1. 18 J g^-1 K^-1
• What assumption is made about the density of solutions in calorimetry?

  Density is assumed to be 1 g cm^-3
• Calculate the energy change for 25 cm³ of solution with a temperature increase of 7.0°C.

  Q = 731.5 J
• How do you calculate the number of moles of CuSO₄ in the example given?

  0.005 mol
• What is the enthalpy change of reaction for the given example?

  146 kJ mol^-1
• Calculate the enthalpy change for the neutralization of HCl.

  • 56.4 kJ mol^-1
• What is the enthalpy change of combustion for propan-1-ol?

  • 1470 kJ mol^-1
• What are the errors in measuring enthalpy changes using calorimetry?

  • Energy loss from surroundings
  • Incomplete combustion of fuel
  • Incomplete energy transfer
  • Evaporation of fuel
  • Heat capacity of calorimeter ignored
  • Non-standard conditions
• What does Hess's Law state?

  Total enthalpy change is independent of reaction route
• What does Hess's Law relate to?

  First law of thermodynamics
• How can different routes of a reaction be represented?

  Using an energy level diagram with arrows
• How can Hess's Law be applied to interconnecting reactions?

  By rearranging equations to find ΔH